1. Field of the Invention
The present invention relates to a color signal transforming apparatus, such as a color printer or a color copying machine that transforms color signals to faithfully reproduce colors from an original document in a color image forming apparatus for outputting signals representative of full colors including half-tone.
2. Description of the Related Art
In the fields of color printing, color television, color copying machine, there are many proposals to transform color signals. In a typical example of those proposals, an input color space, e.g., a BGR color coordinate, is directly transformed into an output color space, e.g., a YMC (K) color coordinate by using a table memory. When three color signals are processed at resolutions of the necessary gray levels, a great amount of table data is required. To store the data, a table memory with a large memory capacity must be provided. Such a memory is very expensive.
For example, in a case where each of the input colors B, G, and R are expressed by 8 bits, and the output colors y, M, C, and K are also expressed by 8 bits, the required memory capacity of the table memory is 2.sup.24 .times.4 bytes. It is impractical to use such a big memory.
Study for reducing the necessary memory capacity in transforming color signals by using the table memory has been made placing an emphasis mainly on the interpolation basis methods.
In these methods, a color correction memory addressed with the higher order bits of the input signals is used for reducing the necessary memory capacity. The coarsened data is corrected by using an interpolation circuit using the lower bits (Published Examined Japanese Patent Application No. Sho. 58-16180 and Published Unexamined Japanese Patent Application No. Hei. 2-187374 may be cited for the methods). Those disclosed techniques still have problems. For example, the interpolation calculations are complicated and consume much time. Further, the adjacent interpolation regions are discontinuous at the boundary.
The methods to solve the problems of the disclosed interpolation techniques have been proposed by the inventor(s) in other co-pending U.S. patent application Ser. Nos. 07/962,502 and 07/962,351 both filed on Oct. 16, 1992, the disclosures of which are incorporated herein by reference. The technical idea of the present patent application is directed to solve the problems of the prior techniques and to further improve the techniques of the above patent applications.
An example of the interpolation method for a 3-dimensional input signals disclosed in the above patent application Ser. No. 07/962,502 will be described with reference to FIG. 13 and using equation (1) below. An example of the interpolation method for a 4-dimensional input signals disclosed in the above patent application Ser. No. 962,351 will be described with reference to FIG. 14 and using an equation (2) below. ##EQU1## where X' (x, y, z) indicates a value of one output in the input (x, y, z). X' (x, y, z, t) indicates a value of one output in the input (x, y, z, t). x.sub.h, y.sub.h, z.sub.h, t.sub.h represent higher bits of an input. x.sub.1, y.sub.1, z.sub.1, t.sub.1 represent lower bits of the same. X'(x.sub.h, y.sub.h, z.sub.h) and X'(x.sub.h, y.sub.h, z.sub.h, t.sub.h) represent each basic data obtained from the combination of the higher bits. a.sub.x (x.sub.h, y.sub.h, z.sub.h, b.sub.x ((x.sub.1, y.sub.1, z.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h, b.sub.y (x.sub.1, y.sub.1, z.sub.1)), a.sub.z(x.sub.h, y.sub.h, z.sub.h, b.sub.z (x.sub.1, y.sub.1, z.sub.1)), or a.sub.x (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.x ((x.sub.1, y.sub.1, z.sub. 1, t.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.y (x.sub.1, y.sub.1, z.sub.1, t.sub.1)), a.sub.z (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.z (x.sub.1, y.sub.1, z.sub.1, h.sub.1)), a.sub.t (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.t (x.sub.1, y.sub.1, z.sub.1, t.sub.1)) represent sensitivity signals for interpolation. b.sub.x ((x.sub.1, y.sub.1, z.sub.1), b.sub.y (x.sub.1, y.sub.1, z.sub.1)), b.sub.z (x.sub.1, y.sub.1, z.sub.1)), or b.sub.x ((x.sub.1, y.sub.1, z.sub.1, t.sub.1)), b.sub.1 (x.sub.1, y.sub.1, z.sub.1, t.sub.1)), b.sub.z (x.sub.1, y.sub.1, z.sub.1, h.sub.1)), b.sub.t (x.sub.1, y.sub.1, z.sub.1, t.sub.1) represent interpolation-region select signals obtained from the combinations of the lower bits.
The arrangements and operations of the color signal transforming apparatus of FIGS. 13 and 14 will be briefly described. Three or four input signals are each divided into higher bits and lower bits. The combinations of the higher bits are input as address signals to a color correction memory 1 for basic data. When receiving the higher bits, the color correction memory 1 outputs X'(x.sub.h, y.sub.h, z.sub.h) in the equation (1) or X'(x.sub.h, y.sub.h, z.sub.h, t.sub.h) in the equation (2).
The combinations of lower bits of the input signals are input to a plural number of interpolation-region select means, which in turn output a plural number of interpolation-region select signals corresponding to b.sub.x (x.sub.1, y.sub.1, z.sub.1), b.sub.y (x.sub.1, y.sub.1, z.sub.1) b.sub.z (x.sub.1, y.sub.1, z.sub.1) in the equation (1) or b.sub.x (x.sub.1, y.sub.1, z.sub.1, t.sub.1), b.sub.y (x.sub.1, y.sub.1, z.sub.1, t.sub.1), b.sub.z (x.sub.1, y.sub.1, z.sub.1, t.sub.1), b.sub.t (x.sub.1, y.sub.1, z.sub.1, t.sub.1) in the equation (2).
The combinations of the higher bits of the input signals and some of the interpolation-region select signals are input to an interpolation sensitivity signal output memory 5 for interpolation. When receiving those signal, the memory 5 produces a plural number of sensitivity signals for interpolation, which represent a.sub.x (x.sub.h, y.sub.h, z.sub.h, b.sub.x ((x.sub.1, y.sub.1, z.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h, b.sub.y (x.sub.1, y.sub.1, z.sub.1)), a.sub.z (x.sub.h, y.sub.h, z.sub.h, b.sub.z (x.sub.1, y.sub.1, z.sub.1)) in the equation (1) or a.sub.x (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.x ((x.sub.1, y.sub.1, z.sub.1, t.sub.1)), a.sub.y (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.y (x.sub.1, y.sub.1, z.sub.1, t.sub.1)), a.sub.z (x.sub.h, y.sub.h, z.sub.h, t.sub.h, b.sub.z (x.sub.1, y.sub.1, z.sub.1, h.sub.1)), a.sub.t (x.sub.h, y.sub.h, z.sub.h, t.sub.1, b.sub.t (x.sub.1, y.sub.1, z.sub.1, t.sub.1)) in the equation (2).
Each sensitivity output signal is multiplied by the corresponding one lower bit of the input signal by each interpolation multiplier 6 as shown. As the results of the multiplications, the multipliers output plural interpolation data.
Finally, adders add together the basic data and the plural interpolation data to produce interpolated values corresponding to X' (x, y, z) in the equation (1) or X' (x, y, z, t) in the equation (2).
The technique has the following features.
(1) There is no need of the complicated address translation when data is read out of the memory. This implies that a high speed processing is possible with a simple circuit construction.
(2) The regular arrangement in the color correction memory is not essential. The technique is compatible with the above-mentioned techniques, which each remove the memory portion corresponding to the gamut by arranging irregularly the data that are regularly arranged in the memory, disclosed in Published Unexamined Japanese Patent Application Nos. Hei. 2-73779 and 2-187374.
(3) A continuity at the boundary region between the adjacent interpolation regions is secured.
It is required to further reduce the memory capacity required in transforming color signals. The above-mentioned techniques, which is for reducing the memory capacity required in transforming color signals, may be further improved in reducing the required memory capacity in the following points.
(1) The technique, which removes the memory portion out of the color reproduction range of the output by arranging irregular the data that are regularly arranged in the memory, disclosed in Published Unexamined Japanese Patent Application No. Hei. 2-187374, needs some 2-dimensional memories. Accordingly, there is room for further improvement of the memory capacity in this technique.
(2) In the techniques disclosed in the above patent application Ser. Nos. 07/962,502 and 07/962,351, the color space is uniformly divided into segmental color spaces. The whole color space is not always equally processed, but the regions having similar color transforming characteristics may be roughly divided.